Electrical power transmission networks are protected and controlled by circuit breakers. Such circuit breakers are divided into two classes: Live tank circuit breakers where the enclosure that contains the breaking mechanism is at line potential, that is, “Live”, and Dead tank circuit breakers where the enclosures are at earth potential.
A circuit breaker for high voltage applications comprises at least one breaking unit and a supporting insulator having one end adapted for connection to a ground potential and the other end adapted for connection to a high voltage potential. The supporting insulator is mechanically connected to the high potential end of the breaking unit(s). The breaking unit includes an interruption chamber housing a movable and a stationary contact.
There is a need to monitor different operating parameters of the circuit breaker, such as the current conducted by the circuit breaker. To this end, it is known to provide the circuit breaker with some kind of measuring device. One kind of measuring device that has come into use recently is the so-called fiber optic current sensor, which works according to the following principles. Magnetic field through a medium changes the polarization of light. By conducting light from the medium by means of an optical fiber to an analyzer and analyzing the polarization, the magnetic field, which is directly proportional to current flowing through the medium, can be determined with high accuracy.
Such arrangements are well known for instance in WO 00/08664 and U.S. Ser. No. 368,567. In all these cases the optical sensors are placed inside the interruption chamber of the circuit breaker. However circuit breakers need maintenance, such as replacing the contacts. It is then necessary to disassemble the circuit breaker. Fiber optic current sensors, which are placed inside the interruption chambers will then be affected. Sometimes the entire breaking units are replaced. Furthermore, large circuit breakers need to be transported separated in parts and the breaking unit will be assembled to the supporting insulator in the switchyard. The optic fibers then must be assembled in the switchyard, often outdoors, which is extremely inconvenient and can jeopardize the accuracy of the fiber optic current sensor.
It is therefore an advantage if the fiber optic current sensor can be placed in a position where its optical connection will not at all be affected when the breaking unit is disassembled from or assembled to the supporting insulator.
GB864,835 shows a circuit breaker having an electrical current sensor, including a current transformer, for measuring the current through the circuit breaker. The current transformer is provided in the lower end of the supporting insulator, which is adapted for connection to a ground potential. The current from the breaking unit is led through the entire supporting insulator to the current sensor located in the lower end of the supporting structure. This is a disadvantage since it requires full high voltage insulation between the primary current path in the circuit breaker and a lower flange in the lower part of the supporting insulator. This full insulation is expensive and complicates the manufacturing of the circuit breaker.